fix(coderd/x/chatd): show correct provider and clean detail for Bedrock errors (#26338)

## Problem
Bedrock errors (routed through aibridge) showed the wrong provider
("Anthropic ...")
and a doubly-wrapped detail string instead of the clean message.

## Fix (chatd only)
- **Provider label:** thread the configured provider to error
classification via
`GenerateAssistantOptions.ErrorProvider`. Transport provider still
drives prompt
  prep, sanitization, and metric labels (unchanged).
- **Detail:** unwrap the SDK transport wrapper (`METHOD "URL": NNN
{body}`) to surface
the inner message; handles top-level `{"message":...}` and nested
`{"error":{"message":...}}`.

## Notes
- Surfacing a top-level `message` now applies to all providers
(intentional; nested wins when both present).
- The advisor path keeps the transport label; accepted as-is (it returns
`err.Error()`, not the classification).
- Fixes display in chatd only; the wrapper originates in aibridge (out
of scope here).

🤖 Generated by Coder Agents.

coderd/x/chatd/chatloop/chatloop.go

@@ -1,6 +1,7 @@
 package chatloop
 
 import (
+	"cmp"
 	"context"
 	"database/sql"
 	"encoding/base64"
@@ -210,7 +211,13 @@ type RunOptions struct {
 
 // GenerateAssistantOptions configures one assistant model call.
 type GenerateAssistantOptions struct {
-	Model                fantasy.LanguageModel
+	Model fantasy.LanguageModel
+	// ErrorProvider labels user-facing errors with the configured provider
+	// identity (e.g. "bedrock"). It differs from Model.Provider(), which
+	// reflects the fantasy transport client and is "anthropic" for Bedrock
+	// routed through aibridge. Metrics and prompt preparation keep using
+	// Model.Provider(). When empty, Model.Provider() is used.
+	ErrorProvider        string
 	Messages             []fantasy.Message
 	Tools                []fantasy.AgentTool
 	ActiveTools          []string
@@ -344,6 +351,11 @@ func GenerateAssistant(ctx context.Context, opts GenerateAssistantOptions) (Assi
 
 	provider := opts.Model.Provider()
 	modelName := opts.Model.Model()
+	// errorProvider labels user-facing errors with the configured provider;
+	// see GenerateAssistantOptions.ErrorProvider. The transport provider is
+	// kept for prompt preparation, Anthropic history sanitization, and the
+	// metric labels below.
+	errorProvider := cmp.Or(opts.ErrorProvider, provider)
 	runOpts := RunOptions{
 		Model:  opts.Model,
 		Logger: opts.Logger,
@@ -382,8 +394,8 @@ func GenerateAssistant(ctx context.Context, opts GenerateAssistantOptions) (Assi
 		opts.Metrics,
 	)
 	if streamErr != nil {
-		wrappedErr := wrapProviderStreamError(provider, streamErr)
-		classified := chaterror.Classify(wrappedErr).WithProvider(provider)
+		wrappedErr := wrapProviderStreamError(errorProvider, streamErr)
+		classified := chaterror.Classify(wrappedErr).WithProvider(errorProvider)
 		if classified.Retryable {
 			opts.Metrics.RecordStreamRetry(provider, modelName, classified)
 		}
@@ -396,8 +408,8 @@ func GenerateAssistant(ctx context.Context, opts GenerateAssistantOptions) (Assi
 		if errors.Is(err, ErrInterrupted) {
 			return AssistantOutcome{}, ErrInterrupted
 		}
-		wrappedErr := wrapProviderStreamError(provider, err)
-		classified := chaterror.Classify(wrappedErr).WithProvider(provider)
+		wrappedErr := wrapProviderStreamError(errorProvider, err)
+		classified := chaterror.Classify(wrappedErr).WithProvider(errorProvider)
 		if classified.Retryable {
 			opts.Metrics.RecordStreamRetry(provider, modelName, classified)
 		}